1. Field of the Invention
The present invention relates to an ink jet recording apparatus for performing the recording by discharging ink onto a recording medium.
2. Related Background Art
Traditionally, there has been known an ink jet recording apparatus for performing the recording by discharging ink droplets onto a recording medium (in most cases, paper, or OHP sheet, cloth, and the like) from a discharging port. The ink jet recording apparatus is a non-impact type recording apparatus capable of performing recording with less noise directly on an ordinary paper as well as the recording of a color image with ease using multicolor. With these features, the widespread use of ink jet recording apparatus has increased rapidly in recent years. Particularly, an ink jet recording apparatus of a type that ink droplets are discharged by an action caused by a phase change generated by the thermal energy given to ink on the basis of recording signals is simple in its structure and has an advantage that a high-precision multinozzle is easily configured to implement a high-resolution and high-speed recording.
However, these ink jet recording apparatuses discharge ink droplets directly from fine discharging ports provided on a surface (discharging surface) of the recording head facing a recording medium. Accordingly, in order to perform a desirable recording, appropriate care should be taken. For example, there is a need for the maintenance of a constant distance between the recording head and recording medium as well as the accurate control of the conveyance of the recording medium. To this end, the recording medium may be electrostatically attracted to a belt or the like which functions as a means for conveying the recording medium. For such a method of conveying the recording medium, there is known a method such as disclosed in Japanese Patent Laid-Open Application No. 62-147473 wherein a belt is charged in advance, and the recording medium is allowed to touch this belt to be attracted thereto by the attraction generated by dielectric polarization, and others.
Furthermore, examples of using static electricity dually as a source to generate energy for discharging ink are disclosed in Japanese Patent Laid-Open Application No. 60-46257, Japanese Patent Laid-Open Application No. 62-151348, and Japanese Patent Laid-Open Application No. 62-225353. In all of these examples, the electrode is arranged on the reverse side of the recording medium (the side at which no recording head is provided) to apply voltage between this electrode and the recording ink.
In the ink jet recording apparatus wherein the recording medium is attracted and held by static electricity according to the conventional art set forth above, an electric field is generated between the surfaces of the recording medium and recording head, and the flight of the ink droplets discharged from the recording head is disturbed. Thus a problem is encountered that the recording is not performed as desired in some cases.
More specifically, satellites (sub-droplets) produced when the ink droplet is split in flying may make a U-turn and sometimes adhere to the vicinity of the discharging port of the discharging surface. The satellites tend to be charged with the same polarity as the recording medium, and it becomes easier for them to adhere to the vicinity of the discharging port of the discharging surface. In other words, the amount of the flying ink toward the recording medium becomes smaller in the case where no electric field mentioned above exists as shown in FIG. 13A, i.e., as compared with the case where no static electricity is used for attracting and holding the recording medium. Further, as shown in FIG. 13B, there is a case where the satellites (sub-droplets) produced due to the splitting of the ink droplet in flight are caused to adhere to the vicinity of the discharging port of the discharging surface because of the aforesaid electric field. If the satellites adhere to the vicinity of the discharging port of the discharging surface like this, the subsequent normal discharging is hindered, leading to the distorted ink flight or disabled ink discharging. If any aqueous ink is employed, it is possible to prevent the adhesion of the satellites to a certain extent by giving a water splashing treatment to the discharging surface, but using only with the water splashing treatment, no sufficient effect is obtainable.
Now, using the drawings, a specific description will be made.
In FIG. 14, the conventional example of the aforesaid ink jet recording apparatus is shown.
In this ink jet recording apparatus, a voltage of approximately +2 kv is applied from a power source 52 to a charging roller 54, and when the charging roller is in contact with a conveyer belt 51 which is means for conveying the recording medium 50, the aforesaid conveyer belt 51 is charged positively (+). When the recording medium 50 is fed onto the aforesaid charged conveyer belt 51 by a carrier roller 53, the aforesaid recording medium 50 is attracted and held by static electricity of the conveyer belt 51 to the conveyer belt 51 and carried in the direction indicated by arrow A. At this juncture, the recording medium 50 is grounded through a resilient electrode 56 provided to be in contact with the recording medium 50 which is being conveyed on the conveyer belt 51. Then, the recording medium 50 is more intensely attracted and held by the conveyer belt 51 to be carried to a position facing the four recording heads 57. Subsequently, ink, colored respectively black, yellow, magenta, and cyan, is discharged from each of the recording heads 57 (57Bk, 57y, 57m, and 57c) to perform the recording on the recording medium 50.
In the aforesaid conventional ink jet recording apparatus, a phase of approximately +800 v exists on the surface of the recording medium 50 according to an experiment. Therefore, as shown in FIGS. 15A through 15D respectively, the ink droplet discharged from each of the recording heads 57 (57Bk, 57y, 57m, and 57c) is polarized and split into the main droplet and satellites (sub-droplets) ultimately in some cases. Here, the satellites are in most cases charged with the same polarity as the recording medium 50 (FIG. 15C). Then, the positively charged satellite repels the recording medium 50 which is given positive charge, and tends to adhere easily to the vicinity of the discharging port 30 of the discharging surface 31 of each of the recording heads 57. Thus, if the satellite adheres to the aforesaid discharging surface 31, a normal discharging is hindered, and there is a possibility that ink cannot be discharged sometimes. Also, in general, the faster the conveying velocity of the recording medium is, the more become the adhesion of the satellites conspicuous, leading to the difficulty in making the recording faster.
Also, particularly, the aforesaid adhesion of the satellites is quite conspicuous in using the full-line head provided with a plurality of discharging ports over the entire width of the recording area as shown in FIG. 15 as described earlier or in color recording.
Subsequently, in this respect, the specific description will be made of the phenomena of the ink adhesion to the vicinity of the discharging port using FIGS. 15A through 15D.
FIG. 15A is a view illustrating the timing immediately before the formation of a discharged droplet. A charging roller 54 made of dielectric rubber to which a voltage of approximately +2 kv has been applied (by a high-voltage power source 52) is brought into contact with a conveyer belt 51 to charge the surface of the conveyer belt 51 with positive charge. Then, by placing the recording medium 50 closely onto the conveyer belt 51, negative charge is given to the side of the recording medium 50 facing the conveyer belt 51. Thus, the attraction of the recording medium 50 and conveyer belt 51 is generated. To the side of the recording medium 50 opposite to the conveyer belt 51 (the side facing the recording heads 57 (57Bk, 57y, 57m, and 57c)), positive charge is given, and a potential difference is generated between the recording heads 57 (57Bk, 57y, 57m, and 57c) and the recording medium 50 to form an electric field. Then, to the liquid column 60 formed by the bubble generated by the thermal driving of the electrothermal converter 40 in the recording head 57 (57Bk, 57y, 57m, and 57c), the negative charge opposite to the positive charge on the recording medium 50 is given, and the droplet 61 is polarized by the effect of the aforesaid electric field as shown in FIG. 15B which represents the phenomenon in the timing for the droplet 61 to fly in the air.
The phenomenon in the next timing is shown in FIG. 15C. As shown in FIG. 15C, the liquid column is split into the main droplet 62 and satellite 63-1 respectively charged negatively and the satellites 63-2 charged positively. Then as shown in FIG. 15D, the main droplet 62 having a large kinetic energy is impacted on the recording medium 50. However, the positively charged satellites repel the positively charged recording medium 50 to adhere to the vicinity of the discharging port 30 by returning in the direction toward the discharging surface 31 in a U-turn fashion as shown in FIG. 15D. This brings about the aforesaid problem.